High strength steel plate having low yield ratio excellent in terms of strain ageing resistance, method of manufacturing the same and high strength welded steel pipe made of the same

ABSTRACT

A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a bainite phase and island martensite, and a polygonal ferrite in surface portions within 5 mm from the upper and lower surfaces, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 μm or less, the area fraction of the polygonal ferrite in the surface portions is 10% to less than 80%.

TECHNICAL FIELD

This disclosure relates to a steel plate having a low yield ratio, highstrength and high toughness preferably used mainly in a linepipe field,a method of manufacturing the steel plate and a high strength weldedsteel pipe, in particular, to a steel plate having a low yield ratio,high strength and high toughness excellent in terms of strain ageingresistance, a method of manufacturing the steel plate and a highstrength welded steel pipe excellent in terms of buckling resistance andductility which is made of the steel plate.

BACKGROUND

Nowadays, steel materials for welded structures are required to have notonly high strength and high toughness, but also a low yield ratio andhigh uniform elongation from the viewpoint of earthquake resistance.Generally, it is known that, by forming a metallographic structure ofthe steel material in which hard phases such as a bainite phase(hereinafter, also referred to as β) and a martensite phase(hereinafter, also referred to as M) are appropriately dispersed in aferrite phase (hereinafter, referred to as a) which is a soft phase, itis possible to achieve a decrease in the yield ratio of a steel materialand an increase in the uniform elongation of a steel material.

As an example of manufacturing methods of forming a microstructure inwhich hard phases are appropriately dispersed in a soft phase asdescribed above, a certain method is described in Japanese UnexaminedPatent Application Publication No. 55-97425. That is, JP '425 disclosesa heat treatment method in which, as an intermediate treatment betweenquenching (hereinafter, also referred to as Q) and tempering(hereinafter, also referred to as T), quenching starting from atemperature range for forming a dual phase consisting of a ferrite phaseand an austenite phase (hereinafter, also referred to as γ) is performed(hereinafter, also referred to as Q′).

Japanese Unexamined Patent Application Publication No. 55-41927discloses an example of a method which does not require additionalmanufacturing processes, in which, after rolling has been finished at atemperature equal to or higher than the Ar₃ transformation point, thestart of accelerated cooling is delayed until the steel material has atemperature equal to or lower than the Ar₃ transformation point at whicha ferrite phase is formed.

As an example of a technique with which it is possible to achieve adecrease in yield ratio without performing complicated heat treatmentsas disclosed in JP '425 and JP '927, Japanese Unexamined PatentApplication Publication No. 1-176027 discloses a method with which adecrease in yield ratio is achieved by finishing rolling of a steelmaterial at a temperature equal to or higher than the Ar₃ transformationpoint, and then by controlling an accelerated cooling rate and a coolingstop temperature to form a dual-phase structure consisting of anacicular ferrite phase and a martensite phase.

Moreover, Japanese Patent No. 4066905 discloses an example of atechnique with which it is possible to achieve a low yield ratio andexcellent toughness in a weld heat affected zone without significantlyincreasing the contents of alloying chemical elements of a steelmaterial, in which, by controlling Ti/N and a Ca—O—S balance, athree-phase structure consisting of a ferrite phase, a bainite phase andisland martensite (hereinafter, also referred to as MA) is formed.

In addition, Japanese Unexamined Patent Application Publication No.2008-248328 discloses a technique in which a decrease in yield ratio andan increase in uniform elongation are achieved by adding alloyingchemical elements such as Cu, Ni and Mo.

On the other hand, in a welded steel pipe used to form a linepipe suchas a UOE steel pipe or an electric resistance welded steel pipe, after asteel plate has been formed by cold forming into a cylindrical shape andthe butted portions have been welded, usually, a coating treatment suchas polyethylene coating or powder epoxy coating is performed on theouter surface of the resultant steel pipe from the viewpoint of, forexample, corrosion protection. Therefore, there is a problem in that,since strain ageing occurs due to working strain applied when pipeforming is performed and due to heating when the coating treatment isperformed, there is an increase in yield stress, which results in theyield ratio of the steel pipe being larger than that of the steel plate.

To solve the problem described above, for example, Japanese UnexaminedPatent Application Publication Nos. 2005-60839 and 2005-60840 disclosesteel pipes having a low yield ratio, high strength and high toughnessexcellent in terms of strain ageing resistance and methods ofmanufacturing the steel pipes utilizing the fine precipitations ofcomplex carbides containing Ti and Mo or the fine precipitations ofcomplex carbides containing two or all of Ti, Nb, and V.

In the heat treatment method according to JP '425, by appropriatelyselecting a quenching temperature in a range forming a dual phase, it ispossible to achieve a decrease in yield ratio, but there is a problem inthat there is a decrease in productivity and there is an increase inmanufacturing cost due to an increase in the number of heat treatmentprocesses.

In addition, in the technique according to JP '927, there is a problemin that there is a significant decrease in productivity, since it isnecessary to perform cooling at a cooling rate equivalent to aspontaneous cooling rate in a temperature range from a rolling finishtemperature to an accelerated cooling start temperature.

Moreover, in the technique according to JP '027, as indicated by theexamples in the literature, there is a problem in that, since the carboncontent or the contents of other alloying chemical elements of a steelplate are increased to obtain a steel material having a tensile strengthof 490 N/mm² (50 kg/mm²) or more, there is an increase in material costand, in addition to that, there is a decrease in toughness in a weldheat affected zone (HAZ).

In addition, in the technique according to JP '905, the influence of,for example, a microstructure on uniform elongation which a steel plateis required to have when the steel plate is used for, for example, alinepipe has not necessarily been clarified. In addition, since thelow-temperature toughness of a base metal was evaluated only at atemperature of −10° C., it is not clear whether or not the steel platecan be used for a new use application in which toughness at a lowertemperature is required.

In the technique according to JP '328, there is a problem in that thereis an increase in material cost, and, in addition to that, there is adecrease in toughness in a weld heat affected zone, since it isnecessary that the steel plate has a chemical composition containingincreased contents of alloying chemical elements. In addition, thelow-temperature toughness of a base metal and a weld heat affected zoneare evaluated only at a temperature of −10° C.

In the techniques according to JP '839 and JP '840, there is an increasein strain ageing resistance, however, the low-temperature toughness of abase metal and a weld heat affected zone are evaluated only at atemperature of −10° C.

Moreover, in JP '425, JP '927, JP '027, JP '905, JP '328, JP '839 and JP'840, it is necessary to form a ferrite phase, but the ferrite phasecauses a decrease in tensile strength. Therefore, it is necessary to addalloying chemical elements to increase strength to an X60 grade or moreaccording to the API standard, and there is concern that there may be anincrease in alloy cost and there may be a decrease in low-temperaturetoughness.

Therefore, it could be helpful to provide a steel plate having a lowyield ratio, high strength and high toughness excellent in terms ofstrain ageing resistance of an API 5L X70 grade or less which can bemanufactured at high productivity, a method of manufacturing the steelplate, and a high strength welded steel pipe made of the steel plate.

SUMMARY

We thus provide:

-   -   [1] A high strength steel plate having a low yield ratio, the        steel plate having a chemical composition containing, by mass %,        C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0%        or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less,        S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and        0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010%        or less, O: 0.005% or less and the balance being Fe and        inevitable impurities, a metallographic structure including a        bainite phase and island martensite, and further including a        polygonal ferrite in surface portions within 5 mm from the upper        and lower surfaces, in which the area fraction of the island        martensite is 3% or more and 15% or less, in which the        equivalent circle diameter of the island martensite is 3.0 μm or        less, in which the area fraction of the polygonal ferrite in the        surface portions is 10% or more and less than 80%, and in which        the remainder of the metallographic structure is a bainite        phase, a hardness variation in the thickness direction of ΔHV30        or less in terms of Vickers hardness, a hardness variation in        the width direction of ΔHV30 or less in terms of Vickers        hardness, a maximum hardness in the surface portions of the        steel plate of HV230 or less in terms of Vickers hardness and a        yield ratio of 85% or less and an elongation of 22% or more in a        full-thickness tensile test using a test piece having a shape in        accordance with GOST standards.    -   [2] The high strength steel plate having a low yield ratio        according to item [1], the steel plate having the chemical        composition further containing, by mass %, one or more selected        from among Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less,        Mo: 0.5% or less, V: 0.1% or less, Ca: 0.0005% or more and        0.003% or less and B: 0.005% or less.    -   [3] A method of manufacturing a high strength steel plate having        a low yield ratio, the method including heating steel having the        chemical composition according to item [1] or [2] at a        temperature of 1000° C. or higher and 1300° C. or lower,        performing hot rolling under conditions such that the cumulative        rolling reduction ratio is 50% or more in a temperature range of        900° C. or lower, finishing hot rolling at a temperature equal        to or higher than the Ar₃ transformation point, starting cooling        when the temperature of the surface portions within 5 mm from        the upper and lower surfaces is equal to or higher than (the Ar₃        transformation temperature −60° C.) and equal to or lower than        the Ar₃ transformation point, performing cooling at a cooling        rate of 200° C./sec. or less in terms of the surface temperature        of the steel plate until the surface temperature becomes 600° C.        or lower, performing cooling at a cooling rate of 15° C./sec. or        more in terms of the average temperature of the steel plate        until the average temperature becomes 450° C. or higher and        650° C. or lower, and, immediately after the cooling has been        performed, performing reheating at a heating rate of 1.0°        C./sec. or more in terms of the surface temperature of the steel        plate until the surface temperature becomes 550° C. or higher        and 750° C. or lower.    -   [4] A high strength welded steel pipe, the steel pipe being        manufactured by forming the steel plate according to item [1] or        [2] into a cylindrical shape, by welding butted portions of the        shaped cylinder under conditions such that single-layer welding        is performed on each of the outer and inner surfaces, and by        thereafter performing a pipe-expanding treatment under a        condition of a pipe expanding ratio of 0.4% or more and 2.0% or        less, and having a yield ratio of 90% or less and an elongation        of 20% or more in a full-thickness tensile test using a test        piece having a shape in accordance with GOST standards, and,        further having a yield ratio of 90% or less and an elongation of        20% or more even after a strain ageing treatment has been        performed under conditions such that the temperature is 250° C.        or lower and the duration is 30 minutes or less.

A steel plate having a low yield ratio, high strength and high toughnessexcellent in terms of strain ageing resistance can be manufacturedwithout decreasing toughness in a weld heat affected zone or adding alarge amount of alloying chemical elements. Therefore, a steel plateused mainly for a linepipe can be stably manufactured in a large amount,and there is a significant increase in productivity and economicefficiency. Moreover, since a high strength welded steel pipe excellentin terms of buckling resistance and ductility can be manufactured usingthis steel plate, there is a significant industrial effect.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram illustrating a thermal history to which asteel plate is subjected after accelerated cooling has been started, inwhich a solid line represents the average temperature of the steel plateand a dot-dash line represents the surface temperature of the steelplate.

DETAILED DESCRIPTION

We conducted investigations regarding a method of manufacturing a steelplate, in particular, regarding a manufacturing process includingcontrolled rolling, accelerated cooling after controlled rolling hasbeen performed and subsequent reheating. We found that, by forming apolygonal ferrite phase only in the surface portion of a steel plate andby controlling rolling conditions so that a bainite phase in the surfaceportion of the steel plate becomes soft, it is possible to achieve highdeformation performance and high ductility without a significantdecrease in strength, and obtained the following findings:

-   -   (a) By stopping accelerated cooling in the middle of an        accelerated cooling process in which a steel plate undergoes        bainite transformation, that is, in a temperature range in which        a non-transformed austenite phase is present, and by        subsequently starting reheating from a temperature higher than a        temperature at which bainite transformation finishes        (hereinafter, referred to as a Bf point), a microstructure of        the metallographic structure of the steel plate, in which hard        island martensite is uniformly formed in a bainite phase, is        formed, and thus there is a decrease in yield ratio.    -   (b) By adding an appropriate amount of Mn to the chemical        composition of steel as a chemical element for stabilizing an        austenite phase, a non-transformed austenite phase become        stabilized and, thus, it is possible to form hard MA without        adding a large amount of chemical elements for increasing        hardenability such as Cu, Ni, and Mo.    -   (c) By performing rolling under conditions such that the        cumulative rolling reduction ratio is 50% or more in a        temperature range of 900° C. or lower, which is a        no-recrystallization temperature range in austenite, fine MA can        be uniformly dispersed, and thus there is an increase in uniform        elongation while a low yield ratio is maintained.    -   (d) By appropriately controlling both the rolling conditions in        the no-recrystallization temperature range in austenite        described in item (c) above and the reheating conditions        described in item (a) above, the shape of MA can be controlled.        That is, the grain size of MA can be controlled to be as small        as 3.0 μm or less in terms of average equivalent circle        diameter. As a result, since MA is only slightly decomposed even        if steel is subjected to a thermal history which causes        deterioration of a yield ratio due to ageing in the case of a        conventional steel, the desired microstructure and properties        can be maintained even after ageing has occurred.    -   (e) By starting the cooling of a steel plate when the        temperature of surface portions within 5 mm from the upper and        lower surfaces of the steel plate is equal to or higher than        (the Ar₃ transformation temperature −60° C.) and equal to or        lower than the Ar₃ transformation temperature, a polygonal        ferrite phase of an appropriate area ratio can be formed in the        surface portions within 5 mm from the upper and lower surfaces.        As a result, since there is a decrease in the hardness of the        surface portions described above, high ductility can be        achieved.    -   (f) By performing a first stage cooling at a cooling rate of        200° C./sec. or less until the temperature of a steel plate        becomes 600° C. or lower, a bainite structure in the surface        portions can be softened. As a result, since there is a decrease        in hardness of the surface portions of a steel plate, high        ductility can be achieved.

The reason for limitations on the features will be described hereafter.

1. Regarding Chemical Composition

First, the reason for the limitations on the chemical composition of thesteel will be described. % used when describing a chemical compositionalways represents mass %.

C: 0.03% or more and 0.08% or less

C is a chemical element which contributes by forming carbide toprecipitation strengthening and is important to form MA. When the Ccontent is less than 0.03%, the content is not sufficient to form MA andsufficient strength cannot be achieved. When the C content is more than0.08%, there is a decrease in the toughness of a base metal andtoughness in a weld heat affected zone (HAZ). Therefore, the C contentis 0.03% or more and 0.08% or less, preferably 0.04% or more and 0.06%or less.

Si: 0.01% or more and 1.0% or less

Si is added to perform deoxidation. When the Si content is less than0.01%, there is an insufficient effect of deoxidation and, when the Sicontent is more than 1.0%, there is a decrease in toughness andweldability. Therefore, the Si content is 0.01% or more and 1.0% orless, preferably 0.01% or more and 0.3% or less.

Mn: 1.2% or more and 3.0% or less

Mn is added to increase strength and toughness. Mn is also added toincrease hardenability that promotes formation of MA. When the Mncontent is less than 1.2%, these effects cannot be sufficiently obtainedand, when the Mn content is more than 3.0%, there is a decrease intoughness and weldability. Therefore, the Mn content is 1.2% or more and3.0% or less, preferably 1.8% or more and 3.0% or less to stably form MAregardless of variations in a chemical composition and manufacturingconditions.

P: 0.015% or less

P is an inevitable impurity, and the upper limit of the P content isspecified. When the P content is large, there is a significant increasein the degree of central segregation, resulting in a decrease in thetoughness of a base metal. Therefore, the P content is 0.015% or less,preferably 0.010% or less.

S: 0.005% or less

S is an inevitable impurity, and the upper limit of the S content isspecified. When the S content is large, there is a significant increasein the amount of MnS formed, resulting in a decrease in the toughness ofa base metal. Therefore, the S content is 0.005% or less, preferably0.002% or less.

Al: 0.08% or less

Al is added as a deoxidation agent. When the Al content is more than0.08%, there is a decrease in the cleanliness of steel, resulting in adecrease in toughness. Therefore, the Al content is 0.08% or less,preferably 0.01% or more and 0.08% or less, more preferably 0.01% ormore and 0.05% or less.

Nb: 0.005% or more and 0.07% or less

Nb is a chemical element which increases toughness as a result ofdecreasing the grain size in the microstructure and contributes to anincrease in strength due to an increase in hardenability through the useof solute Nb. These effects are obtained when the Nb content is 0.005%or more. However, when the Nb content is more than 0.07%, there is adecrease in toughness in a weld heat affected zone, and thus the Nbcontent is 0.005% or more and 0.07% or less, more preferably 0.01% ormore and 0.05% or less.

Ti: 0.005% or more and 0.025% or less

Ti is an important chemical element which increases the toughness of abase metal as a result of preventing an increase in the grain size of anaustenite phase through the use of the pinning effect of TiN when slabheating is performed. This effect is obtained when the Ti content is0.005% or more. However, when the Ti content is more than 0.025%, thereis a decrease in toughness in a weld heat affected zone, and thus the Ticontent is 0.005% or more and 0.025% or less, preferably 0.005% or moreand less than 0.02% from the view point of toughness in a weld heataffected zone, more preferably 0.007% or more and 0.016% or less.

N: 0.010% or less

N is treated as an inevitable impurity. Since there is a decrease intoughness in a weld heat affected zone when the N content is more than0.010%, the N content is 0.010% or less, preferably 0.007% or less, morepreferably 0.006% or less.

O: 0.005% or less

O is an inevitable impurity and the upper limit of the O content isspecified. Since O causes formation of coarse inclusions which has anegative effect on toughness, the O content is 0.005% or less, morepreferably 0.003% or less.

The basic chemical composition is as described above. Furthermore, toimprove the strength and toughness of a steel plate and to increasehardenability to promote formation of MA, one or more of Cu, Ni, Cr, Mo,V, Ca, and B described below may be added.

Cu: 0.5% or less

Since Cu contributes to an increase in the hardenability of steel whenCu is added, Cu may be added. It is preferable that the Cu content be0.05% or more to obtain this effect. However, when the Cu content is0.5% or more, there is a decrease in toughness. Therefore, when Cu isadded, it is preferable that the Cu content be 0.5% or less, morepreferably 0.4% or less.

Ni: 1% or less

Since Ni contributes to an increase in the hardenability of steel and,in particular, since there is not a decrease in toughness even when theNi content is large, Ni is effective to increase toughness. Therefore,Ni may be added. It is preferable that the Ni content be 0.05% or moreto obtain this effect. However, since Ni is an expensive chemicalelement, when Ni is added, it is preferable that the Ni content be 1% orless, more preferably 0.4% or less.

Cr: 0.5% or less

Since Cr is, like Mn, a chemical element effective to achieve sufficientstrength even when the C content is low, Cr may be added. Although it ispreferable that the Cr content be 0.1% or more to obtain this effect,there is a decrease in weldability when the Cr content is excessivelylarge. Therefore, when Cr is added, it is preferable that the Cr contentbe 0.5% or less, more preferably 0.4% or less.

Mo: 0.5% or less

Since Mo is a chemical element which increases hardenability, and sinceMo is a chemical element which contributes to an increase in strength asa result of formation of MA and strengthening a bainite phase, Mo may beadded. It is preferable that the Mo content be 0.05% or more to obtainthis effect. However, when the Mo content is more than 0.5%, there is adecrease in toughness in a weld heat affected zone. Therefore, when Mois added, it is preferable that the Mo content be 0.5% or less, morepreferably 0.3% or less.

V: 0.1% or less

Since V is a chemical element which contributes to an increase instrength as a result of increasing hardenability, V may be added. It ispreferable that the V content be 0.005% or more to obtain this effect.However, when the V content is more than 0.1%, there is a decrease intoughness in a weld heat affected zone. Therefore, when V is added, itis preferable that the V content be 0.1% or less, more preferably 0.06%or less.

Ca: 0.0005% or more and 0.003% or less

Since Ca is a chemical element which increases toughness as a result ofcontrolling the shape of inclusions of sulfide, Ca may be added. Thiseffect is realized when the Ca content is 0.0005% or more. When the Cacontent is more than 0.003%, this effect becomes saturated and there isconversely a decrease in toughness as a result of decreasingcleanliness. Therefore, when Ca is added, it is preferable that the Cacontent be 0.0005% or more and 0.003% or less, more preferably 0.001% ormore and 0.003% or less. B: 0.005% or less

Since B is a chemical element which contributes to an increase instrength and an increase in toughness in a weld heat affected zone, Bmay be added. It is preferable that the B content be 0.0005% or more toobtain these effects. However, when the B content exceeds 0.005%, thereis a decrease in weldability. Therefore, when B is added, it ispreferable that the B content be 0.005% or less, more preferably 0.003%or less.

By controlling the ratio of the Ti content and the N content Ti/N, it ispossible to prevent an increase in the grain size of an austenite phasein a weld heat affected zone as a result of presence of TiN particles,and it is possible to achieve good toughness in a weld heat affectedzone. Therefore, it is preferable that Ti/N be 2 or more and 8 or less,more preferably 2 or more and 5 or less.

The remainder of the chemical composition other than those describedabove in a steel plate consists of Fe and inevitable impurities.However, as long as the desired effects are not decreased, chemicalelements other than those described above may be added. For example,from the viewpoint of increasing toughness, Mg: 0.02% or less and/or REM(rare-earth metal): 0.02% or less may be added.

Subsequently, the metallographic structure will be described.

2. Regarding Metallographic Structure

The metallographic structure is controlled so that a duplex-phasestructure consisting of a bainite phase and island martensite is a mainbody of the metallographic microstructure and areas (surface portions)within 5 mm from the upper and lower surfaces have a three-phasestructure consisting of a polygonal ferrite, a bainite phase, and islandmartensite.

By forming a duplex-phase structure serving as a main body of thestructure, in which hard MA is included in a bainite phase, a decreasein yield ratio, an increase in uniform elongation, and an increase instrength are obtained. In addition, by forming a three-phase structureincluding a polygonal ferrite phase in surface portions within 5 mm fromthe upper and lower surfaces, an increase in uniform elongation and anincrease in ductility are obtained.

When steel plates and steel pipes are used in regions such as earthquakeregions in which a large deformation is applied to these materials,these materials may be required to have not only a low yield ratio butalso high uniform elongation and high ductility. In multi-phasestructures consisting of a soft polygonal ferrite, a bainite phase andhard MA as described above, the soft phase undergoes deformation. Inaddition, by controlling hardness variations in the thickness directionand in the width direction to be about ΔHV30 or less and a maximumhardness in the surface portions of the steel plate to be about HV230 orless, it is possible to achieve a high elongation of 22% or more in afull-thickness tensile test using a test piece having a shape inaccordance with GOST standards for a steel plate.

The content ratio of MA in a metallographic structure is 3% or more and15% or less in terms of area fraction of MA (calculated as the averagevalue of the area ratios of MA in arbitrary cross sections in therolling direction, the width direction and the like of a steel plate).When the area fraction of MA is less than 3%, there may an insufficientdecrease in yield ratio. When the area fraction of MA is more than 15%,there may be a decrease in the toughness of a base metal.

In addition, it is preferable that the area fraction of MA be 5% or moreand 15% or less from the viewpoint of decreasing yield ratio andincreasing uniform elongation and base-material toughness. The areafraction of MA means the ratio with respect to the whole microstructureof steel.

MA can be easily identified by observing a sample prepared by etching asteel plate using, for example, a 3% nital solution (nital: nitric acidalcohol solution) and by subsequently performing electrolytic etching.By observing the microstructure of a steel plate using a scanningelectron microscope (SEM), MA is recognized as a distinct white portion.

The area fraction of MA can be calculated, for example, as the averagevalue of the area ratios constituted by MA by performing imageprocessing on microstructure photographs of at least 4 microscopicfields taken through observation using a scanning electron microscope(SEM).

In addition, the equivalent circle diameter of MA is 3.0 μm or less fromthe viewpoint of achieving sufficient toughness for a base metal andincreasing uniform elongation. This is because, when the equivalentcircle diameter of MA is more than 3.0 μm, there may be a decrease inthe toughness of a base metal.

The equivalent circle diameter of MA can be calculated as the averagevalue of the diameters of the circles respectively having the same areasas MA grains obtained by performing image processing on microstructuretaken through observation using a SEM.

Mn and Si are added to form MA without adding large amounts of expensivealloying chemical elements such as Cu, Ni, and Mo. It is important tostabilize a non-transformed austenite phase with this method to suppresspearlite transformation or formation of a cementite phase whenair-cooling is performed after reheating has been performed.

The mechanism to form MA and a polygonal ferrite phase in the upper andlower surface portions will be roughly described hereafter.Manufacturing conditions will be described in detail later.

After having heated a slab, hot rolling is finished in a temperaturerange for forming an austenite phase, and then accelerated cooling isstarted at a temperature just below the Ar₃ transformation temperature.

In a manufacturing process, in which, after accelerated cooling has beenfinished in the middle of a bainite transformation process, that is, ina temperature range in which a non-transformed austenite phase ispresent, reheating is started at a temperature higher than a temperatureat which bainite transformation (Bf point) is finished, and then coolingis performed, changes in a microstructure will be described hereafter.

When accelerated cooling is finished, the microstructure consists of apolygonal ferrite phase in the upper and lower surface portions, abainite phase, and a non-transformed austenite phase. After that, byperforming reheating starting at a temperature higher than the Bf point,transformation from a non-transformed austenite phase to a bainite phaseoccurs and, since the solid solubility limit of C in a bainite phase issmall, C is evacuated into the surrounding non-transformed austenitephase.

Therefore, the C content in a non-transformed austenite phase increasesas bainite transformation progresses when reheating is performed. Atthis time, when austenite stabilizing chemical elements such as Cu andNi are added in certain amounts or more, a non-transformed austenitephase in which C is concentrated are retained even after reheating hasbeen finished and transforms into MA when cooling is performed afterreheating has been performed and, fin-ally, a microstructure in which MAis formed in a bainite structure is formed. The microstructure furtherincluding a polygonal ferrite phase is formed in the upper and lowersurface portions.

The area fraction of a polygonal ferrite phase in the surface portionsis 10% or more and less than 80%. This is because, when the areafraction of an polygonal ferrite phase in the surface portions is lessthan 10%, the surface of a steel plate has an excessively high hardnessof more than HV230, resulting in a case where elongation is less than22%. In addition, this is because, when the area fraction of a polygonalferrite phase in the surface portions is 80% or more, there is anexcessive decrease in strength of a steel plate.

It is important to start reheating in a temperature range in which anon-transformed austenite phase is present after accelerated cooling hasbeen performed. When the reheating-start temperature is equal to orlower than the Bf point, bainite transformation is completed, and anon-transformed austenite phase is not present. Therefore, it isnecessary that the reheating-start temperature be higher than the Bfpoint.

In addition, there is no particular limitation on what cooling method isused after reheating has been performed because it has no influence onMA transformation. However, it is basically preferable that air-coolingbe used. By using steel containing a certain amount of Mn, by stoppingaccelerated cooling in the middle of a bainite transforming process andby subsequently starting reheating immediately after the cooling hasbeen stopped, hard MA can be formed without decreasing productivity.

Although the steel plate has a metallographic structure in which acertain amount of MA is uniformly included in a bainite phase and inwhich a polygonal ferrite phase is further included in the surfaceportions within 5 mm from the upper and lower surfaces, this disclosureincludes a steel plate containing other types of microstructures andprecipitations as long as the desired effects are not decreased.

Specifically, when one, two or more of other types of microstructuressuch as a pearlite phase and a cementite phase are formed together inaddition, there is a decrease in strength. However, when the areafractions of the microstructures other than a polygonal ferrite phase, abainite phase and MA is small, an effect of decreasing strength isnegligible. Therefore, as long as the total area fraction of ametallographic structures other than the three kinds of microstructures,which are a polygonal ferrite phase, a bainite phase and MA, is 3% orless with respect to the whole microstructure, one or more of themetallographic structures such as a pearlite phase, a cementite phaseand the like may be included.

The metallographic structure described above can be formed using steelhaving the chemical composition described above and the manufacturingmethods described hereafter.

3. Regarding Manufacturing Conditions

It is preferable that steel having the chemical composition describedabove be refined with a common method using a refining means such as aconverter furnace or an electric furnace and be made into a steelmaterial such as a slab using a common method such as a continuouscasting method or an ingot casting-slabbing method. The refining methodand a casting method are not limited to those described above. Afterthat, the slab is rolled into a desired shape, and cooling and heatingare performed after rolling has been performed.

The cooling start temperature is expressed in terms of the surfacetemperature of a steel plate, and a cooling rate and a cooling stoptemperature are expressed in terms of both the surface temperature of asteel plate and the average temperature of a steel plate, unlessotherwise noted. Other temperatures such as a slab heating temperature,a controlled rolling start temperature, a controlled rolling finishtemperature and a reheating temperature in a reheating process areexpressed in terms of the average temperature of a steel plate.

The average temperature of a steel plate is calculated from the surfacetemperature of a slab or a steel plate in consideration of parameterssuch as thickness and thermal conductivity. In addition, the coolingrate is an average cooling rate derived by dividing a temperaturedifference necessary in a cooling process after hot rolling has beenfinished down to a cooling stop temperature (450° C. to 650° C.) by atime spent for the cooling.

In addition, the heating rate is an average heating rate derived bydividing a temperature difference necessary in a reheating process aftercooling has been finished up to a reheating temperature (550° C. to 750°C.) by a time spent for the reheating. Manufacturing conditions will bedescribed in detail hereafter.

As the Ar₃ temperature, the value derived using the following equationwill be used:

Ar₃(° C.)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo,

where an atomic symbol represents the mass % of the chemical elementrepresented by the symbol.Heating temperature: 1000° C. or higher and 1300° C. or lower

When the heating temperature is lower than 1000° C., a sufficientsolution of carbides cannot be achieved, and the desired strength cannotbe achieved. In the case where the heating temperature is higher than1300° C., there is a decrease in the toughness of a base metal.Therefore, the heating temperature is 1000° C. or higher and 1300° C. orlower.

Rolling finish temperature: equal to or higher than the Ar₃ temperature

When the rolling finish temperature is lower than the Ar₃ temperature,there is a decrease in ferrite transformation speed after rolling hasbeen finished, and there is insufficient concentration of C in anon-transformed austenite phase when reheating is performed, whichresults in MA not being formed. Therefore, the rolling finishtemperature is equal to or higher than the Ar₃ temperature.

Cumulative rolling reduction ratio in a temperature range of 900° C. orlower: 50% or more

A temperature range of 900° C. or lower corresponds to the lower part ofa no-recrystallization temperature range in austenite. By controllingthe cumulative rolling reduction ratio to be 50% or more in thistemperature range, a decrease in austenite grain size can be achieved.With this method, there is an increase in the number of MA forming sitesat prior-austenite grain boundaries afterward, which contributes to thesuppression of an increase in MA grain size.

When the cumulative rolling reduction ratio in a temperature range of900° C. or lower is less than 50%, the equivalent circle diameter offormed MA becomes more than 3.0 μm, resulting in a case where there is adecrease in uniform elongation and/or there is a decrease in thetoughness of a base metal. Therefore, the cumulative rolling reductionratio in a temperature range of 900° C. or lower is 50% or more.

FIG. 1 is a schematic diagram illustrating a cooling curve expressed interms of the average temperature of a steel plate and a cooling-heatingcurve expressed in terms of the surface temperature of a steel plate inan accelerated cooling process.

A cooling start temperature is equal to or higher than (the Ar₃transformation temperature −60° C.) and equal to or lower than the Ar₃transformation temperature in terms of the surface temperature of asteel plate. This condition is one of the important manufacturingconditions. By controlling the accelerated cooling start temperatureafter rolling has been finished to be equal to or lower than the Ar₃transformation temperature and equal to or higher than (the Ar₃transformation temperature −60° C.) in terms of the surface temperatureof a steel plate, a polygonal ferrite can be formed in an amount of 10%or more and less than 80% in terms of area fraction in surface portionswithin 5 mm from the upper and lower surfaces of the steel plate. As aresult, the maximum hardness of the surface portions of the steel platecan be controlled to be HV230 or less. Thus, the elongation of the steelplate can be controlled to be 22% or more in a full-thickness tensiletest using a test piece having a shape in accordance with GOSTstandards.

Regarding the cooling conditions of the steel plate, the cooling rate is200° C./sec. or less and the cooling stop temperature is 600° C. orlower in terms of the surface temperature of the steel plate.

By performing cooling under conditions such that the cooling rate is200° C./sec. or less and the cooling stop temperature is 600° C. orlower, a bainite structure in the surface portions of the steel platebecomes soft, and thus the maximum hardness of the surface portions ofthe steel plate can be controlled to be HV230 or less in terms ofVickers hardness, and hardness variations in the thickness direction andin the width direction can be suppressed to be ΔHV30 or less. Bycontrolling hardness variations in the thickness and width directions tobe ΔHV30 or less and by controlling the maximum hardness of the surfaceportions to be HV230 or less as described above, the elongationvariation and a decrease in elongation of the steel plate can besuppressed, and an elongation of 22% or more can be stably achieved.

In addition, the reason why the yield ratio of a steel plate is 85% orless and the elongation of the steel plate is 22% or more will bedescribed hereafter. This is because it is necessary that the targetvalue of the yield ratio be 85% or less and the target value of theelongation be 22% or more at the stage of the steel plate in order toachieve a yield ratio of 90% or less and an elongation of 20% or more atthe stage of a steel pipe in consideration of changes in the materialproperties due to the working strain induced by forming a steel plate toa steel pipe.

The cooling stop temperature is 600° C. or lower in terms of the surfacetemperature of the steel plate to control the surface temperature of thesteel plate to be lower than or equal to a temperature at which bainitetransformation starts, and it is preferable that the cooling stoptemperature be 200° C. or higher and 500° C. or lower in terms of thesurface temperature of the steel plate. With this method, themetallographic structure of surface portions within 5 mm from the upperand lower surfaces of the steel plate becomes a three-phase structureconsisting of a polygonal ferrite phase, a bainite phase and MA.Incidentally, it is preferable that the lower limit of the cooling ratebe 50° C./sec.

A cooling rate in terms of the average temperature of the steel plate is15° C./sec. or more.

When the cooling rate is less than 15° C./sec., a pearlite phase isformed when cooling is performed, and sufficient strength orsufficiently low yield ratio cannot be achieved. Therefore, the coolingrate in terms of the average temperature of the steel plate is 15°C./sec. or more.

The steel plate is supercooled to a temperature range for bainitetransformation by performing accelerated cooling, and it is possible tocomplete bainite transformation when reheating is subsequently performedwithout holding the steel plate at the reheating temperature.

A cooling stop temperature in terms of the average temperature of thesteel plate is 450° C. or higher and 650° C. or lower.

This process is an important manufacturing condition. A non-transformedaustenite phase present after reheating has been performed and in whichC is concentrated, transforms into MA when air-cooling is performedafter reheating has been performed.

That is, it is necessary that cooling be stopped in the middle ofbainite transformation, that is, in a temperature range in which anon-transformed austenite phase is present. When the cooling stoptemperature is lower than 450° C., bainite transformation is completed,and MA is not formed when air-cooling is performed and a decrease inyield ratio cannot be achieved. When the cooling stop temperature ishigher than 650° C., C is consumed by a pearlite phase which isprecipitated when cooling is performed, and MA is not formed. Therefore,the accelerated cooling stop temperature is 450° C. or higher and 650°C. or lower. The accelerated cooling stop temperature preferably be 500°C. or higher and 600° C. or lower from the viewpoint of achieving thearea fraction of MA which is ideal for achieving better strength andtoughness. Regarding this accelerated cooling, an arbitrary coolingapparatus can be used.

Immediately after accelerated cooling has been stopped, reheating isperformed up to a temperature of 550° C. or higher and 750° C. or lowerat a heating rate of 1.0° C./sec. or more in terms of the surfacetemperature of the steel plate.

“Reheating is performed immediately after accelerated cooling has beenstopped” means that reheating is performed at a heating rate of 1.0°C./sec. or more within 120 seconds after accelerated cooling has beenstopped.

This process is also an important manufacturing condition. While anon-transformed austenite phase transforms into a bainite phase whenreheating is performed after accelerated cooling has been performed asdescribed above, C is evacuated into a non-transformed austenite phasewhich is remaining. Then, the non-transformed austenite phase in which Cis concentrated is transformed into MA when air-cooling is performedafter reheating has been performed.

To form MA, it is necessary that reheating be performed from atemperature higher than the Bf point to a temperature of 550° C. orhigher and 750° C. or lower after accelerated cooling has beenperformed.

When the heating rate is less than 1.0° C./sec., it takes a long timefor a steel plate to reach the target reheating temperature, whichresults in a decrease in productivity and, there may be an increase inMA grain size. As a result, it is impossible to achieve a sufficientlylow yield ratio, sufficient toughness or sufficient uniform elongation.Although the mechanisms are not necessarily clear, the reason is thoughtto be as follows. That is, by increasing the heating rate for reheatingto be 1.0° C./sec. or more, an increase in grain size in a region inwhich C is concentrated is suppressed, and an increase in MA grain sizewhich is formed in a cooling process after reheating has been performedis suppressed.

When the reheating temperature is lower than 550° C., transformationdoes not sufficiently progress, and a sufficient amount of C is notevacuated into a non-transformed austenite phase and a decrease in yieldratio cannot be achieved as a result of a sufficient amount of MA notbeing formed. When the reheating temperature is higher than 750° C.,sufficient strength cannot be achieved due to the softening of a bainitephase. Therefore, the reheating temperature is 550° C. or higher and750° C. or lower.

After accelerated cooling has been performed, it is important to startreheating in a temperature range in which a non-transformed austenitephase is present. In this reheating process, when a reheating starttemperature is equal to or lower than the Bf point, bainitetransformation is completed, and a non-transformed austenite phasedisappears. Therefore, it is necessary that the reheating starttemperature be higher than the Bf point.

To ensure that C is concentrated in a non-transformed austenite phase,it is preferable that a steel plate be heated up to a temperature 50° C.or more higher than the reheating start temperature. It is notparticularly necessary that a holding time be set during which the steelplate is held at the reheating temperature.

By using the manufacturing method, even when cooling is performedimmediately after reheating has been performed, a sufficient amount ofMA is achieved. As a result, a decrease in yield ratio and an increasein uniform elongation are achieved. However, to ensure that sufficientvolume fraction of MA is achieved by promoting the diffusion of C into anon-transformed austenite phase more, the steel plate may be held at thereheating temperature for 30 minutes or less.

When the temperature holding time is more than 30 minutes, there may bea decrease in strength due to the occurrence of recovery of a bainitephase. In addition, it is fundamentally preferable to perform coolingusing an air-cooling method after reheating has been performed.

As an apparatus to perform reheating after accelerated cooling has beenperformed, a heating apparatus may be equipped downstream of a coolingapparatus to perform accelerated cooling. Among heating apparatuses, itis preferable to use a gas-fired furnace or an induction heatingapparatus with which a steel plate can be heated at a high heating rate.

As described above, first, rolling is performed under conditions suchthat the cumulative rolling reduction ratio is 50% or more in atemperature range of 900° C. or lower, which is a no-recrystallizationtemperature range in austenite. With this method, there is an increasein the number of MA formation sites as a result of a decrease inaustenite grain size, and fine MA can be uniformly dispersed, whichresults in a low yield ratio of 85% or less in the state of a steelplate and of 90% or less in the state of a steel pipe being achieved.

Moreover, by performing reheating at a high heating rate afteraccelerated cooling has been performed, an increase in MA grain size issuppressed so that the equivalent circle diameter of MA is decreased to3.0 μm or less. In addition, by starting cooling at a temperature equalto or higher than (the Ar₃ transformation temperature −60° C.) and equalto or lower than the Ar₃ transformation temperature, a polygonal ferritephase is formed in the surface portions within 5 mm from the upper andlower surfaces, and, by performing cooling at a cooling rate of 200°C./sec. or less in terms of the surface temperature of a steel platedown to a temperature of 600° C. or lower in terms of the surfacetemperature of a steel plate, a bainite phase in the surface portion ofthe steel plate can be softened, which results in an elongation of 22%or more in the state of a steel plate and of 20% or more in the state ofa steel pipe being achieved in a full-thickness tensile test using atest piece having a shape in accordance with GOST standards.

With this method, even if a thermal history which causes deteriorationof properties due to strain ageing in the case of a conventional steelis applied, MA is less likely to decompose in the steel, and it ispossible to maintain the specified metallographic structure in which aduplex-phase structure consisting of a bainite phase and MA is mainlyincluded and surface portions within 5 mm from the upper and lowersurfaces have a three-phase structure consisting of a polygonal ferritephase, a bainite phase, and MA.

As a result, even if a thermal history is applied at a temperature of250° C. for 30 minutes, which is classified into a thermal history at ahigh temperature and for a long duration among common coating processesfor steel pipes, an increase in yield stress (YS) that is caused bystrain aging and accompanied by an increase in yield ratio and adecrease in uniform elongation can be suppressed. That is, the specifiedproperties in the state of a steel plate and in the state of a steelpipe can be assured in the steel even if a thermal history which causesdeterioration of properties due to strain ageing in a conventional steelplate is applied.

When a steel pipe is manufactured using the steel plate, the steel plateis formed into a cylindrical shape and the butted portions thereof arewelded under conditions such that single-layer welding is performed oneach of the outer and inner surfaces. Then, by performing apipe-expanding treatment under a condition of a pipe expanding ratio of0.4% or more and 2.0% or less, a steel pipe having good roundness can beobtained.

Examples 1

The steels (steel types A through J) having the chemical compositionsgiven in Table 1 were cast into slabs using a continuous casting method,and thick steel plates (Nos. 1 through 17) having a thickness of 20 mm,28 mm or 33 mm were manufactured.

TABLE 1 Steel Chemical Composition (mass %) Type C Si Mn P S Al Nb Ti CuNi Cr Mo A 0.032 0.20 2.5 0.008 0.001 0.03 0.034 0.014 — — — — B 0.0510.56 1.8 0.008 0.002 0.04 0.023 0.011 0.24 0.20 — — C 0.072 0.06 1.80.011 0.001 0.03 0.044 0.013 — — — 0.22 D 0.064 0.15 1.7 0.008 0.0010.03 0.021 0.009 0.20 0.20 — 0.18 E 0.054 0.15 2.2 0.008 0.001 0.040.025 0.008 — — 0.10 — F 0.058 0.16 1.7 0.009 0.001 0.03 0.009 0.0160.16 0.18 0.03 0.20 G 0.063 0.13 1.9 0.008 0.001 0.03 0.014 0.013 — — —0.20 H 0.023 0.38 2.4 0.008 0.002 0.03 0.032 0.010 — — — — I 0.062 0.651.1 0.009 0.001 0.03 0.024 0.011 — — — 0.10 J 0.071 0.34 2.2 0.008 0.0010.03 0.035 0.014 — — — — Ar₃ Transformation Steel Chemical Composition(mass %) Point Type V Ca B N O (° C.) Ti/N Note A — — — 0.004 0.002 7003.5 Example B — — — 0.005 0.001 734 2.2 Example C — — — 0.004 0.001 7263.3 Example D — 0.0018 — 0.005 0.002 725 1.8 Example E — — — 0.005 0.002716 1.6 Example F 0.030 0.0016 — 0.006 0.002 725 2.7 Example G — —0.0010 0.004 0.002 722 3.3 Example H — — — 0.005 0.001 711 2.0Comparative Example I — — 0.0008 0.004 0.002 795 2.8 Comparative ExampleJ — — — 0.004 0.002 734 3.5 Comparative Example Annotation: Underlinedvalue is out of our range. Annotation: Ar₃ Transformation Point (° C.) =910-310C—80Mn—20Cu—15Cr—55Ni—80Mo (An atomic symbol represents thecontent (mass %) of the chemical element represented by the symbol.)

Immediately after hot-rolling had been performed on the heated slabs,cooling was performed using a water-cooling type accelerated coolingapparatus, and then reheating was performed using an induction heatingfurnace or a gas-fired furnace. The induction heating furnace wasequipped on the same line as an accelerated cooling apparatus.

The manufacturing conditions of the steel plates (Nos. 1 through 17) aregiven in Table 2. Temperature such as a heating temperature, a rollingfinish temperature, a cooling stop (finish) temperature, and a reheatingtemperature were represented in terms of the average temperature of thesteel plate. The average temperature was calculated from the surfacetemperature of the slab or the steel plate using parameters such asthickness and thermal conductivity.

In addition, a cooling rate is an average cooling rate which was derivedby dividing a temperature difference necessary in a cooling processafter hot rolling has been finished down to a cooling stop (finish)temperature (430° C. to 630° C.) by a time spent for the cooling. Inaddition, reheating rate (heating rate) is an average heating rate whichwas derived by dividing a temperature difference necessary in areheating process after cooling had been finished up to a reheatingtemperature (530° C. to 680° C.) by a time spent for the reheating.

TABLE 2 Cooling in Terms of Surface Temperature*1 Cooling StopTemperature Cumulative Cooling of Steel Rolling Start Cooling PlateReduction Temperature Rate of Surface Ratio Rolling of Steel Steel(Before Heating at 900° C. Finish Plate Plate Heat Steel ThicknessTemperature or under Temperature Surface Surface Recovery) No. Type (mm)(° C.) (%) (° C.) (° C.) (° C./sec.) (° C.) 1 A 20 1130 65 760 680 130390 2 B 20 1120 60 740 700 135 430 3 C 33 1080 70 750 710 120 410 4 D 201180 70 750 700 140 420 5 E 28 1050 60 740 690 135 340 6 F 33 1150 55750 710 130 400 7 G 28 1150 75 770 720  95 370 8 E 20  970 75 750 690135 410 9 E 20 1150 40 730 700 140 380 10 E 20 1180 75 760 680 130 40011 F 28 1100 65 740 700 135 260 12 F 28 1200 60 790 690 135 410 13 F 281080 70 760 720 140 350 14 F 28 1080 70 760 720 230 200 15 H 20 1150 75760 700 140 420 16 I 20 1090 70 820 740 140 310 17 J 28 1180 75 760 690135 380 Cooling in Terms of Average Temperature*2 Average AverageCooling Cooling Stop Rate Temperature of Steel of Steel ReheatingReheating Plate Plate Reheating Rate Temperature No. (° C./sec.) (° C.)Apparatus (° C./sec.) (° C.) Note  1 30 590 Induction   1.2 650 ExampleHeating Furnace  2 35 630 Induction 3 650 Example Heating Furnace  3 20610 Induction 3 680 Example Heating Furnace  4 40 620 Induction 5 650Example Heating Furnace  5 35 540 Gas-fired 2 680 Example Furnace  6 30600 Induction 3 660 Example Heating Furnace  7 35 570 Induction 5 650Example Heating Furnace  8 35 610 Induction 7 680 Comparative HeatingExample Furnace  9 40 580 Induction 5 650 Comparative Heating ExampleFurnace 10  3 600 Induction 6 680 Comparative Heating Example Furnace 1135 430 Induction 5 650 Comparative Heating Example Furnace 12 35 610Induction   0.2 680 Comparative Heating Example Furnace 13 40 550Induction 7 530 Comparative Heating Example Furnace 14 40 550 Induction7 650 Comparative Heating Example Furnace 15 40 620 Induction 6 650Comparative Heating Example Furnace 16 40 510 Induction 7 680Comparative Heating Example Furnace 17 35 580 Induction 2 650Comparative Heating Example Furnace Annotation: Underlined value is outof our range. *1Cooling is controlled on the basis of a surfacetemperature of a steel plate. *2Cooling is controlled on the basis of anaverage temperature of a steel plate.

The mechanical properties of the steel plate manufactured as describedabove were determined. The results are given in Table 3. Tensilestrength was evaluated on the basis of the average value thereof derivedby collecting 2 test pieces for a full-thickness tensile test in adirection (C direction) at a right angle to the rolling direction and byperforming a tensile test. The required strength is a tensile strengthof 590 MPa or more. A yield ratio and an elongation were evaluated bycollecting test pieces for a full-thickness tensile test in a direction(C direction) at a right angle to the rolling direction and byperforming a tensile test. The required deformation performances are ayield ratio of 85% or less, a uniform elongation of 10% or more and atotal elongation of 22% or more.

The toughness of a base metal was evaluated, by collecting 3 full-size Vnotch Charpy test pieces in a direction at a right angle to the rollingdirection, by performing a Charpy test, by determining absorbed energyat a temperature of −40° C. and by calculating the average value of the3 values. When the absorbed energy at a temperature of −40° C. was 200 Jor more was evaluated as good.

Regarding toughness in a weld heat affected zone (HAZ), 3 test piecesthat had been subjected to a thermal history corresponding to a heatinput of 70 kJ/cm using a reproducing apparatus of weld thermal cycleswere collected, and a Charpy impact test was performed for those testpieces. Then, absorbed energy at a temperature of −40° C. wasdetermined, and the average value for the 3 test pieces was calculated.A case where the Charpy absorbed energy at a temperature of −40° C. was100 J or more was evaluated as good.

TABLE 3 Polygonal Ferrite Area MA Fraction Equivalent Within C Directionin Plate State MA Circle 5 mm from (Before Ageing Treatment FractionDiameter Upper and of 250° C.′ 30 min.) in Steel in Steel Lower TensileUniform Steel Thickness Plate Plate Surfaces Strength Yield RatioElongation No. Type (mm) (%) (mm) (%) (MPa) (%) (%) 1 A 20 11  1.6 60621 75 12 2 B 20 8 1.2 50 610 74 12 3 C 33 13  2.6 40 677 71 11 4 D 20 71.7 50 622 75 13 5 E 28 6 1.6 55 655 73 11 6 F 33 10  1.3 70 662 78 13 7G 28 4 1.5 40 636 70 12 8 E 20 1 2.5 50 556 89  9 9 E 20 7 3.5 55 608 7712 10 E 20 2 2.4 40 620 87 11 11 F 28 0 1.5 60 655 94  9 12 F 28 1 1.635 660 83  8 13 F 28 0 1.3 40 633 89  9 14 F 28 5 1.8 50 645 84 12 15 H20 1 1.4 50 655 88  8 16 I 20 0 1.8 35 581 86  9 17 J 28 14  4.3 40 64366 12 C Direction in Plate State (Before Ageing Treatment of 250° C.′ 30min.) Hardness Variation Hardness Base In Variation Metal HAZ TotalThickness in Width Surface Toughness Toughness Elongation DirectionDirection Hardness vE-40° C. vE-40° C. No. (%) DHV DHV HV (J) (J) Note 1 24 20 25 222 307 141 Example  2 25 15 18 203 312 124 Example  3 23 2423 214 294 118 Example  4 25 22 19 211 274 164 Example  5 26 14 16 220318 155 Example  6 24 18 13 217 333 131 Example  7 25 11 14 216 361 182Example  8 26 22 23 210 335 178 Comparative Example  9 27 25 24 222 129124 Comparative Example 10 24 23 22 211 273 138 Comparative Example 1121 26 34 232 285 161 Comparative Example 12 20 23 16 224 288 144Comparative Example 13 20 25 25 215 312 116 Comparative Example 14 21 3532 235 277 136 Comparative Example 15 25 16 18 207 293 122 ComparativeExample 16 21 22 27 227 281 133 Comparative Example 17 21 34 33 233 302 28 Comparative Example Annotation: Underlined value is out of ourrange. Annotation: Tensile test was performed according to GOST (longgauge length).

In Table 3, in all our examples Nos. 1 through 7, the chemicalcompositions and the manufacturing conditions were in our range, andeach of these examples had a high strength of 590 MPa or more in termsof tensile strength, a low yield ratio of 85% or less, a high uniformelongation of 10% or more, a high total elongation of 22% or more andgood toughness for a base metal and a weld heat affected zone.

In addition, the microstructure in the central portion of the steelplate included mainly a bainite phase in which MA is dispersed, in whichthe area fraction of MA was 3% or more and 15% or less, and in which theequivalent circle diameter of MA was 3.0 μm or less. The area fractionof MA was determined by performing image processing on a microstructureobserved using a scanning electron microscope (SEM). In addition, themicrostructure in the surface proportions of the steel plate includedmainly a polygonal ferrite phase and a bainite phase, in which MA isdispersed, and in which the area fraction of a polygonal ferrite phasewas 10% or more and 80% or less.

On the other hand, in the comparative examples Nos. 8 through 14, thechemical compositions were in our range, but the manufacturing methodswere out of our range. As a result, the microstructures were out of ourrange, and the yield ratio or the elongation was insufficient or thesufficient strength or toughness was not obtained in both conditions ofbefore and after a strain ageing treatment at a temperature of 250° C.for 30 minutes. In Nos. 15 through 17, since the chemical compositionswere out of our range, the yield ratio and uniform elongation of No. 15was out of our range, and the tensile strength, yield ratio, uniformelongation and elongation of No. 16 were all out of our range. Thetoughness in a weld heat affected zone (HAZ) of No. 17 was out of ourrange.

Subsequently, UOE steel pipes were manufactured using the steel plates(Nos. 1 through 17) that had been manufactured under conditions given inTable 2.

After the obtained steel plates were formed by performing U-press andO-press, using a submerged arc welding method, inner surface seamwelding was performed, and then outer surface seam welding wasperformed. Subsequently, by performing pipe-expanding treatment under acondition of a pipe-expanding ratio of 0.6% to 1.2%, steel pipes havingan outer diameter of 400 to 1626 mm were manufactured. Using a tensiletest piece in accordance with GOST standards cut out of the base metalof the steel pipe, tensile properties were evaluated. In addition, usinga tensile test piece in accordance with GOST standards cut out of asample material by the same method described above, which had been cutout of the base metal of the pipe to form the tensile test pieces andsubjected to an ageing treatment at a temperature of 250° C. for 30minutes, tensile properties after having undergone ageing treatment wereevaluated. In addition, using a V-notch Charpy impact test pieceaccording to JIS Z 2202 (1980) cut out of the central portion in thethickness direction of the base metal of the steel pipe, a Charpy impacttest was performed at a test temperature of −40° C. Moreover, using atest piece for a DWTT (Drop Weight Tear Test) according to API-5L cutout of the steel pipe, a DWTT was performed at a test temperature of−20° C. to determine an SA value (Shear Area: ductile fracture arearatio). In addition, using a V-notch Charpy impact test piece accordingto JIS Z 2202 (1980) cut out of the portion of the outer surface FL(Fusion Line) of the welded joint of the steel pipe, a Charpy impacttest was performed at a test temperature of −40° C. The notch was formedat a position where the HAZ and the weld metal were included at a ratioof 1:1.

The test results are given in Table 4.

TABLE 4 Polygonal C Direction in UOE Pipe State MA Ferrite (BeforeAgeing Treatment of 250° C.′ 30 min.) Equivalent Area Fraction Base MACircle within 5 mm Metal Fraction Diameter from Upper Toughness HAZ inSteel in Steel and Lower Yield Tensile Yield Uniform Total DWTTToughness Steel ThickNess Plate Plate Surfaces Strength Strength RatioElongation Elongation vE-40° C. SA-20° C. vE-40° C. No. Type (mm) (%)(mm) (%) (MPa) (MPa) (%) (%) (%) (J) (%) (J) 1 A 20 11  1.6 60 532 62685 10  22 307 100 141 2 B 20 8 1.2 50 517 616 84 10  23 312 100 124 3 C33 13  2.6 40 547 675 81 9 21 294 100 118 4 D 20 7 1.7 50 534 628 85 11 23 274 100 164 5 E 28 6 1.6 55 547 659 83 9 24 318 100 155 6 F 33 10 1.3 70 559 665 84 12  22 333  95 131 7 G 28 4 1.5 40 514 642 80 10  23361 100 182 8 E 20 1 2.5 50 514 559 92 7 24 335  95 178 9 E 20 7 3.5 55509 613 83 10  25 129  75 124 10 E 20 2 2.4 40 571 628 91 9 22 273  95138 11 F 28 0 1.5 60 599 663 90 7 19 285 100 161 12 F 28 1 1.6 35 559666 84 6 18 288 100 144 13 F 28 0 1.3 40 581 639 91 7 18 312 100 116 14F 28 5 1.8 50 554 652 85 11  19 277 100 136 15 H 20 1 1.4 50 599 661 916 23 293  95 122 16 I 20 0 1.8 35 534 580 92 7 19 281  95 133 17 J 2814  4.3 40 518 648 80 10  19 302  60  28 C Direction in Ageing TreatedPipe State (After Ageing Treatment of 250° C.′ 30 min.) Base Uni- MetalL Direction in Ageing Treated Pipe State form Total Toughness HAZ (AfterAgeing Treatment of 250° C.′ 30 min.) Yield Tensile Yield Elonga-Elonga- DWTT Toughness Yield Tensile Yield Uniform Total StrengthStrength Ratio tion tion vE-40° C. SA-20° C. vE-40° C. Strength StrengthRatio Elongation Elongation No. (MPa) (MPa) (%) (%) (%) (J) (%) (J)(MPa) (MPa) (%) (%) (%) Note 1 544 632 86 9 22 321 100 132 542 630 8610  22 Example 2 531 625 85 10  22 304 100 133 530 623 85 11  21 3 575684 84 10  21 288 100 122 571 680 84 10  22 4 532 633 84 11  23 268 100141 529 630 84 10  23 5 557 671 83 9 23 307 100 146 549 661 83 9 24 6584 679 86 10  21 311  95 120 580 674 86 11  22 7 534 651 82 10  22 341100 152 533 650 82 11  22 8 524 570 92 7 23 311  95 141 521 566 92 7 22Com- 9 543 617 88 10  24 134  75 102 540 614 88 9 24 parative 10 538 62686 8 21 266  95 108 536 623  6 8 22 Example 11 620 674 92 5 19 277 100114 617 671 92 6 20 12 569 677 84 6 17 269 100 138 567 675 84 7 18 13588 646 91 6 18 274 100 104 586 644 91 7 19 14 572 657 87 8 18 266 100122 570 655  7 8 18 15 608 675 90 7 22 288  95 133 606 673 90 7 23 16535 588 91 7 18 278  95 103 533 586 91 7 18 17 544 664 82 9 19 245  60 19 542 661 82 9 19 Annotation: Underlined value is out of our range.Annotation: Tensile test was performed according to GOST (long gaugelength).

Our target ranges regarding the base metal of a pipe are: a tensilestrength of 590 MPa or more, an elongation of 20% or more, and a ratioof a 0.5%-proof stress to a tensile strength of 90% or less, where allof those were determined before and after an ageing treatment at atemperature of 250° C. for 30 minutes. At the same time, our targetranges regarding the base metal are a Charpy absorbed energy at a testtemperature of −40° C. of 200 J or more and a DWTTSA −20° C. (ductilefracture area ratio in a DWTT test at a test temperature of −20° C.) of85% or more, and our target range regarding the seam weld joint of steelpipe is a Charpy absorbed energy of 100 J or more at an outer surface FLnotch at a temperature of −40° C.

In Table 4, in all our examples Nos. 1 through 7, the chemicalcompositions and the manufacturing methods were in our ranges.Therefore, these examples had a high tensile strength of 590 MPa ormore, a low yield ratio of 90% or less and a high elongation of 20% ormore before and after a strain ageing treatment at a temperature of 250°C. for 30 minutes, and, further, the toughness of a base metal and in aweld heat affected zone were good.

On the other hand, in the comparative examples Nos. 8 through 14, thechemical compositions were in our range, but the manufacturing methodswere out of our range. As a result, the microstructures were out of ourrange, and the yield ratio or elongation was insufficient or thesufficient strength or toughness was not obtained in both conditions ofbefore and after a strain ageing treatment at a temperature of 250° C.for 30 minutes. In Nos. 15 through 17, since the chemical compositionswere out of our range, the yield ratio and uniform elongation of No. 15were out of our range, and the tensile strength, yield ratio, uniformelongation and elongation of No. 16 were all out of our range. Theelongation and toughness in a weld heat affected zone of No. 17 were outof our range.

1. A high strength steel plate having a low yield ratio, the steel platehaving a chemical composition containing, by mass %, C: 0.03% or moreand 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or moreand 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% orless, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and0.025% or less, N: 0.010% or less, O: 0.005% or less and the balancebeing Fe and inevitable impurities, a metallographic structure includinga bainite phase and island martensite, and further including a polygonalferrite phase in surface portions within 5 mm from the upper and lowersurfaces, wherein an area fraction of island martensite is 3% or moreand 15% or less, wherein an equivalent circle diameter of the islandmartensite is 3.0 μm or less, wherein the area fraction of the polygonalferrite phase in the surface portions is 10% or more and less than 80%,and wherein the remainder of the metallographic structure consists of abainite phase, a hardness variation in a thickness direction of ΔHV30 orless in terms of Vickers hardness, a hardness variation in a widthdirection of ΔHV30 or less in terms of Vickers hardness, a maximumhardness in surface portions of the steel plate of HV230 or less interms of Vickers hardness and a yield ratio of 85% or less and anelongation of 22% or more in a full-thickness tensile test using a testpiece having a shape in accordance with GOST standards.
 2. The steelplate according to claim 1, wherein the chemical composition furthercomprises, by mass %, one or more selected from among Cu: 0.5% or less,Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca:0.0005% or more and 0.003% or less and B: 0.005% or less.
 3. A method ofmanufacturing a high strength steel plate having a low yield ratiocomprising: heating steel having the chemical composition according toclaim 1 at a temperature of 1000° C. or higher and 1300° C. or lower,performing hot rolling under conditions such that cumulative rollingreduction ratio is 50% or more in a temperature range of 900° C. orlower, finishing hot rolling at a temperature equal to or higher thanthe Ar₃ transformation point, starting cooling when the temperature ofthe surface portions within 5 mm from the upper and lower surfaces isequal to or higher than (the Ar₃ transformation temperature −60° C.) andequal to or lower than the Ar₃ transformation point, performing coolingat a cooling rate of 200° C./sec. or less in terms of a surfacetemperature of the steel plate until the surface temperature becomes600° C. or lower, performing cooling at a cooling rate of 15° C./sec. ormore in terms of the average temperature of the steel plate until theaverage temperature becomes 450° C. or higher and 650° C. or lower, and,immediately after the cooling has been performed, performing reheatingat a heating rate of 1.0° C./sec. or more in terms of the surfacetemperature of the steel plate until the surface temperature becomes550° C. or higher and 750° C. or lower.
 4. A high strength welded steelpipe, the steel pipe being manufactured by forming the steel plateaccording to claim 1 into a cylindrical shape, by welding buttedportions of the shaped cylinder under conditions such that single-layerwelding is performed on each of the outer and inner surfaces, and bythereafter performing a pipe-expanding treatment under a condition of apipe expanding ratio of 0.4% or more and 2.0% or less, and having ayield ratio of 90% or less and an elongation of 20% or more in afull-thickness tensile test using a test piece having a shape inaccordance with GOST standards and, further having a yield ratio of 90%or less and an elongation of 20% or more even after a strain ageingtreatment has been performed under conditions such that the temperatureis 250° C. or lower and the duration is 30 minutes or less.
 5. A methodof manufacturing a high strength steel plate having a low yield ratiocomprising: heating steel having the chemical composition according toclaim 2 at a temperature of 1000° C. or higher and 1300° C. or lower,performing hot rolling under conditions such that the cumulative rollingreduction ratio is 50% or more in a temperature range of 900° C. orlower, finishing hot rolling at a temperature equal to or higher thanthe Ar₃ transformation point, starting cooling when the temperature ofthe surface portions within 5 mm from the upper and lower surfaces isequal to or higher than (the Ar₃ transformation temperature −60° C.) andequal to or lower than the Ar₃ transformation point, performing coolingat a cooling rate of 200° C./sec. or less in terms of a surfacetemperature of the steel plate until the surface temperature becomes600° C. or lower, performing cooling at a cooling rate of 15° C./sec. ormore in terms of the average temperature of the steel plate until theaverage temperature becomes 450° C. or higher and 650° C. or lower, and,immediately after the cooling has been performed, performing reheatingat a heating rate of 1.0° C./sec. or more in terms of the surfacetemperature of the steel plate until the surface temperature becomes550° C. or higher and 750° C. or lower.
 6. A high strength welded steelpipe, the steel pipe being manufactured by forming the steel plateaccording to claim 2 into a cylindrical shape, by welding buttedportions of the shaped cylinder under conditions such that single-layerwelding is performed on each of the outer and inner surfaces, and bythereafter performing a pipe-expanding treatment under a condition of apipe expanding ratio of 0.4% or more and 2.0% or less, and having ayield ratio of 90% or less and an elongation of 20% or more in afull-thickness tensile test using a test piece having a shape inaccordance with GOST standards, and, further having a yield ratio of 90%or less and an elongation of 20% or more even after a strain ageingtreatment has been performed under conditions such that the temperatureis 250° C. or lower and the duration is 30 minutes or less.